An ideal analytical micro method permit studies not only of samples of minute volume but also of low concentration. The former requirement is fulfilled by electrophoresis and chromatography in the capillary format, but not always the latter. The reason need not be that the original concentration of the analytes in the sample is extremely low but that the concentration of the analytes decreases drastically due to adsorption onto all surfaces with which they come into contact. However, this adsorption seldom alters the concentration of all analytes to the same degree, and therefore often changes the quantitative composition of the sample. The problem is particularly great with small sample volumes when the concentration of the solutes is so low that an enrichment is required for detection.
Concentration of solutes in small volumes offers special problems. The difficulties are particularly great when the sample concentration is extremely low because the adsorption onto the surfaces with which the solutes come into contact may be very strong. The adsorption not only decreases the original sample concentration but also often alters the relative concentrations of the solute, causing serious deviations from the original composition of the sample.
Bundles of a great number of hollow fibers are used routinely for cross-flow ultra-dialysis both on the laboratory and industrial scale. Such bundles have been employed also for continuous, on-line dialysis of chromatographic eluates against water for desalting against an appropriate solution for buffer exchange and against polyethylene glycol for enrichment of macromolecules from chromatographic columns. Kaufmann, M.; Schwarz, T.; Bartholmes, P. J. Chromatogr. 1993, 639, 33. Hollow fibers can also serve as separation chambers in field-flow-fractionation. Lee, H. L.; Lightfoot, E. N. Sep. Sci. 1976, 11, 417. Jonsson, J. A.; Carlshaf, A. Anal, Chem. 1989, 61, 11. A novel concentration method and apparatus of this invention uses a hollow fiber filled with the dilute sample solution. The technique is based on the transport of water out of the fiber through the pores in its wall and has the advantage that solute adsorption is negligible. Thus, the present invention provides a simple micro method for the concentration of analytes without disturbing adsorption. The method is based on the transport of water by evaporation or the Donnan effect through the pores in the wall of a single hollow fiber filled with the sample. The fiber can be used not only for the concentration of macromolecules but also for low-molecular-weight compounds and inorganic ions. The fiber also permits equilibration of polymer against any solution.